Prime Minister John Key has announced the by-election for the Mt Roskill seat will take place on December 3.
In the wake of his election to Auckland mayor, Phil Goff has resigned from Parliament and his final day is on Tuesday.
Mr Key admitted on Monday that no incumbent Government has ever won a seat they didn't already hold.
"So I think we need to accept that's a bit tricky," Mr Key said.
Mt Roskill has been held by Labour since it was created in 1999 and Michael Wood is standing as Labour's candidate for this by-election.
Mr Key also mentioned the "deal" done between the Green Party and Labour for the seat, which had "added to the complexity and challenge of it".
He said National MP Parmjeet Parmar would be a strong candidate if she put her name forward, while ACT is not standing in the by-election to further National's chances of winning.
The last by-election in New Zealand was in Northland last year, sparked by the resignation of National MP Mike Sabin.
NZ First leader Winston Peters won by a landslide.